Alzheimer disease, which was first described in 1906 by German neuropathologist Alois Alzheimer, is a degenerative and deeply debilitating brain disorder that develops in mid- to late adulthood. An estimated 35.6 million people worldwide were living with dementia in 2010, a figure that was expected to double over the next two decades. Yesterday it was announced that a panel of experts for the National Institute on Aging and the Alzheimer’s Association had developed new guidelines for diagnosing the disease, in an effort to diagnose it early. According to Daniel J. DeNoon of WebMD, this is the first time in 27 years that a change in diagnosis guidelines has been made.

To understand the timing of this decision, we asked Kara Rogers, Britannica’s senior biomedical sciences editor, what prompted the change. She told us:

Histopathologic image of neuritic plaques in the cerebral cortex in a patient with Alzheimer disease of presenile onset, before age 65; KGH

In the mid-1980s, when the original diagnostic criteria were formed, Alzheimer’s was thought to have just one stage—dementia. If people did not have clinical symptoms, namely memory loss and loss of control over body functions, Alzheimer disease wasn’t suspected. In fact, many of the symptoms now known to be linked with Alzheimer’s, including mood swings and subtle changes in cognition, were just thought to be part of the normal aging process. Of course, now, thanks to advances in diagnostic imaging and in the ability to detect biomarkers, which are substances found in blood or spinal fluid that are indicative of disease, three different stages—preclinical, mild cognitive impairment, and Alzheimer’s dementia—can be distinguished. The new diagnostic guidelines cover these stages and, very importantly, provide flexibility for advances in technologies and scientific understanding of the disease.